The present invention relates to an image reading apparatus such as a digital copying machine, and more particularly, improvement of a scanner unit.
An original glass 5 serving as an original-placing portion is formed on the upper surface portion of a copying machine main body. The original glass 5 is covered with an open/close cover 6. When an original 2 such as document is copied, the open/close cover 6 is opened as shown in FIG. 1, and the original 2 is placed on the left-hand side of the original glass 5. At the time, if noise light 7 outside the original glass 5 and light 8 reflected from a light radiation lamp (not shown) enter into a lens unit, a defective image 4, namely, a hollow image, parts of which are faint, is produced on a copying paper sheet, as shown in FIG. 2.
To prevent the ambient light 7 and the reflection light 8 from entering into the lens unit 20, a light-shield cover 40 is attached to the lens unit 20 in a conventional scanner unit 100, as shown in FIGS. 3 and 7. The lens cover 40 covers the lens unit 20 together with a CCD assembly 30.
In the scanner unit 100, a board cover 50 is provided so as to cover a scanner electrical circuit board 55. The board cover 50 is arranged next to the lens cover 40. The upper surfaces of the board cover 50 and the lens cover 40 are placed at the same level (height). A first carriage 70 moves in the space right above both covers 40 and 50. On the other hand, a second carriage (not shown) moves solely in a side space of the lens cover 40 (the left-half side space in FIG. 7). The moving stroke (length) of the second carriage is approximately half the width of the scanner unit 100.
Incidentally, the scanner unit 100 sends the image of the original projected by a mirror (not shown) to a lens 25, through which the image can be read by a CCD 36. To send the image from the lens 25 to the CCD 36 without fail, the lens unit 20 must be securely positioned relative to other optical units. Thus, the lens unit 20 is fitted onto a lens base 10 and has position-adjusting mechanisms 16, 22, 23, 33a and 33b. The lens unit 20 is positioned accurately to other optical units in the directions of an X-axis, Y-axis, and Z-axis by the position adjusting mechanisms 16, 22, 23, 33a and 33b. 
However, the conventional scanner unit 100 has the following problems (1) to (3).
(1) Since the lens cover 40 is used for covering both the lens unit 20 and the CCD assembly 30, and the board cover 50 is used for covering the scanner electrical board 55, the total size of both the covers 40 and 50 approximately equates half the size of the scanner unit 100 (see FIG. 7). The lens cover 40 itself requires sheet-metal working, which incurs costs. In addition, a number of screws 9 are required for fixing the lens cover 40 onto a base frame 80 (FIG. 3). The cost for fitting the lens cover 40 to the base frame 80 must be added. As a result, the total cost becomes high.
(2) Since the lens cover 40 must be removed from the base frame 80 every time rescaling of the CCD 36 is performed and the lens unit 20 is exchanged, time and labor are required for removing the screws 9. Furthermore, since an original detection sensor 44 and a cooling fan 42 are attached to the lens cover 40, harnesses 60 of electrical parts 42 and 44 have to be removed from a harness clamp 61 (see FIG. 3). This requires a great deal of time and labor and thus lowers the serviceability.
(3) The lens cover 40 must have an allowance in height. As the allowance, the margin for a height-adjusting stroke of a CCD board 31+α is required. Due to this, a clearance G1 between the upper surface of the lens cover 40 and the lowermost portion of the first carriage 70 restricted down to 1 to 1.5 mm (see FIG. 7). Since the clearance G1 is extremely narrow, if the lens cover 40 and the first carriage 70 are erroneously fixed at positions far away from the correct positions, the first carriage 70 may hit the lens cover 40 when moved. For this reason, the lens cover 40 and the first carriage 70 must be precisely fixed. On the other hand, when the first carriage 70 is positioned further upward in order to increase the clearance G1, the overall height of the scanner unit 100 increases. As a result, the space occupied by the whole machine increases. This goes against the requirements for miniaturization.